Method of crucible-free zone melting crystalline rods, especially of semiconductor material

ABSTRACT

Method of zone melting a semiconductor rod includes displacing an end holder of the vertically held rod transversely to the rod axis, continuously displacing the other end holder vertically so as to feed a rod portion, having a diameter greater than the inner diameter of an annular heating device surrounding the rod and forming a melting zone therein, into the melting zone, forming a diametrical constriction in the melting zone, displacing the one end holder vertically and rotating it until the rod portion located between it and the melting zone is formed to a specific diameter larger than the inner diameter of the heating device and, after forming the rod portion to the specific cross section, vertically displacing the end holders elative to the heating device.

United States Patent Keller et al. 1 Apr. 25, 1972 54] METHOD OF CRUCIBLE-FREE ZONE [56] References Cited MELTING CRYSTALLINE RODS, UNITED STATES PATENTS ESPECIALLY OF SEMICONDUCTOR 2,972,525 2/l96l Emeis .23/301 MATERIAL Related U.S. Application Data Continuation of Ser. No. 570,503, Aug. 5, 1966, abandoned.

Foreign Application Priority Data Aug. 7, 1965 Germany ..S 98712 U.S. Cl ..148/l.6, 23/273 SP, 23/301 SP Int. Cl. ..B01j [7/02, B01d 9/00 Field of Search 1 48/1 .6; 23/204 R, 273 SP,

FOREIGN PATENTS OR APPLICATIONS 223,659 10/1962 Austria ..l48/l.6

Primary Examiner-Oscar R. Vertiz Assistant Examiner-Hoke S. Miller Att0rneyCurt M. Avery 57 ABSTRACT Method of zone melting a semiconductor rod includes displacing an end holder of the vertically held rod transversely to the rod axis, continuously displacing the other end holder vertically so as to feed a rod portion, having a diameter greater than the inner diameter of an annular heating device surrounding the rod and forming a melting zone therein, into the melting zone, forming a diametrical constriction in the melting zone, displacing the one end holder vertically and rotating it until the rod portion located between it and the melting zone is formed to a specific diameter larger than the inner diameter of the heating device and, after forming the rod portion to the specific cross section, vertically displacing the end holders elative to the heating device.

6 Claims, 3 Drawing Figures Patented April 25, 1972 3,658,598

METHOD OF CRUCIBLE-FREE ZONE MELTING CRYSTALLINE RODS, ESPECIALLY OF SEMICONDUCTOR MATERIAL This is a continuation of application Ser. No. 570,503, filed Aug. 5, 1966, and now abandoned.

Our invention relates to crucible-free zone melting and particularly to method of increasing the cross section of a crystalline rod, especially of semiconductor material, during crucible-free or floating zone melting thereof.

In the copending application of one of the joint applicants herein, namely application Ser. No. 428,933 of W. Keller, filed Jan. 29, 1965, which matured into US. Pat. No. 3,414,388 on Dec. 3, 1968, there is provided in a process of zone melting a semiconductor rod wherein the rod is vertically supported by end holders located in the vertical axis of the rod, and a molten zone is formed in the rod by an annular heating device surrounding and spaced from the rod, the end holders being relatively movable in a direction toward one another and the heating device being displaceable along the rod so as to pass the molten zone along the rod, the improvement which comprises continuously displacing one of the end holders in a direction transverse to the vertical axis of the rod and the annular heating device, also displacing the one end holder in a vertical direction, and rotating the one end holder until the rod portion located between the one end holder and the molten zone is formed to a specific diameter larger than the inner diameter of the annular heating device, then, after forming the rod portion to the specific cross section, displacing the one end holder only in a vertical direction relative to the heating device.

In the aforementioned copending application, it was further disclosed that the rod portion being fed into the melt of the molten zone has a diameter which is smaller than the inner diameter of the annular heating device.

An object of the invention in the instant application is to provide a method for producing crystalline rods of large diameter whose length is greater than the length of the rods produced by the method of the aforementioned copending application.

A further object of our invention is to provide such an improved method as will yield a greater output of recrystallized material since the ends of the rods that are unusable for further processing have the same length regardless of the length of the entire rod, i.e. whether it is long or short.

Yet another object of the invention is to provide an improved method of the foregoing type wherein the rods produced have a cross section even larger than is obtainable by the method of the aforementioned copending application.

With the foregoing and other objects in view, we provide a further improvement in the improved method of the aforementioned copending application which comprises feeding into the melt of the molten zone a portion of the rod having a diameter larger than the inner diameter of the annular heating device, and forming a diametral constriction in the molten zone. Thus, only a relatively small region of the feed rod portion is melted, and that portion of the molten material still remaining above the annular heating device or induction coil is supported or levitated by the magnetic field of the coil windings which are located below so as to prevent dripping of the melt from the molten zone. The supporting action is further increased by providing a heating current of suitable frequency to the heating coil, such as in the range of 500 kilocycles to 2 megacycles; for example, however, the normal frequency of 4 megacycles provides sufiicient supporting action in many cases.

In accordance with a further feature of the invention, the cross section of the rod portion being fed into the molten zone melt is the same as the desired cross section of the crystallized rod portion, whereby the most efficient use is made of the length in an available zone melting apparatus having a given maximum distance between the end rod holders, and a rod of specific diameter can be passed through the heating coil of such zone melting apparatus several times without having its diameter changed.

Other features which are considered as characteristic for the invention are set forth in the appended claims.

Although the invention is illustrated and described herein as method of crucible-free zone melting crystalline rods, especially of semiconductor material, it is nevertheless not intended to be limited to the details shown, since various modifications may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.

The method of our invention, however, together with additional objects and advantages thereof, will be best understood from the following description when read in connection with the accompanying drawings, in which:

FIGS. 1 to 3 are elevational views partly broken away and partly in section showing various phases of the rod and molten zone during the method of the invention.

Referring now to the drawing, and first to FIG. 1 thereof, there is shown a rod 2 of semiconductor material, for example, having a diameter greater than the inner diameter of an induction heating coil 3. At the lower end of the rod 2 surrounded by the annular heating coil 3, a diametral constriction is formed in the molten zone 5 produced by the induction heating coil 3. The constriction may be produced for example by mechanical means such as lapping or sand-blasting the rod at the desired region of constriction, or by chemical means such as etching. In the case at hand, the constriction is produced by fusing to the rods 2 a thin, rod-shaped seed crystal such as a monocrystal used for growing monocrystals.

To produce the molten zone 5, the heating coil 3 is energized by a high frequency current. The seed crystal 4 is rotated about its vertical axis and, at the instant represented in FIG. 1, the molten zone has reached the point at which the transition from thin seed crystal to normal rod thickness is achieved.

In FIG. 2, there is represented a later stage wherein further steps of the method of this invention are indicated by corresponding arrows. The seed crystal 4 is not only displaced downwardly, as viewed in FIG. 2, with respect to the heating coil 3, which is assumed to be stationary, but is also simultaneously displaced with the recrystallized rod portion 2a toward the right-hand side of the figure in the plane of the drawing. The molten zone is consequently also pulled or stretched to the right. The upper rod portion is fed downwardly from about at a suitable rate.

In FIG. 3, the stable finalconditions of the method is shown wherein there is no further lateral or transverse displacement of the seed crystal 4 and the lower rod portion 2a; but rather, both of the rod portions 2 and 2a; and the seed crystal as well, are displaced only downwardly as viewed in FIG. 3.

The method of our invention has been hereinbefore described and illustrated with respect to an example wherein the diameter of the crystallized rod portion is substantially the same as that of the rod portion being fed into the melt of the molten zone, the feed rod is passed downwardly through a stationary heating coil, and the seed crystal fused to the lower end of the rod 2 is rotated about its vertical axis.

It is believed to be understood, however, that it is within the scope and range of equivalence of our invention when the heating coil is displaceable and is passed along the longitudinal axis of a stationary rod portion, when both rod portions are rotated in opposite rotary directions about their longitudinal axis, or when instead of rotating the crystallized rod portion, this motion is simulated by a circular motion of the heating device that corresponds to the motion of an eccentric ring, and also when the seed crystal is fused to the upper end of the rod 2 and the two rod portions are displaced upwardly relative to the heating device. It is furthermore possible, in accordance with the invention of this application, so to move the rod holders and, if necessary, the heating device relative to one another, that the diameter of the crystallizing rod portion is either decreased or further increased.

The crystal quality of the rod portion 2a may be further improved by afterheating the zone crystallizing from the melt by means of an additional annular heating device, such as the heating coil 3a, shown in FIG. 3, thereby rendering the solidification or freezing of the rod portion 2a more uniform. The after heating device may work by radiation, in which case the temperature thereof should be at least the same as the melting temperature of the crystalline material being processed. It may also be advantageous for the rod being fed into the melt of the molten zone to be preheated by still another annular heating device such as the heating coil 3b shown in FIG. 3.

As noted in the aforementioned copending application, the method of this application may be carried out either in vacuum or protective gas atmosphere with apparatus such as is described in great detail in the aforementioned copending application and in US. Pats. Nos. 2,972,525; 2,992,311 and 3,030,194, for example.

We claim:

1. Method of zone melting a semiconductor rod wherein the rod is vertically supported by end holders located in the vertical axis of the rod, and a molten zone is formed in the rod by an annular heating device of given inner diameter surrounding and spaced from the rod, the end holders being relatively movable in a direction toward one another, and the heating device being relatively displaceable with respect to the end holders along the rod so as to pass the molten zone along the rod, which comprises rotating at least one of the end holders about its vertical axis and continuously relatively displacing the one end holder in a vertical direction and also in a direction transverse to the vertical axis of the rod and the annular heating device until the rod portion located between the one end holder and the melting zone is formed to a specific diameter larger than the inner diameter of the annular heating device, continuously relatively displacing the other of the end holders in a vertical direction so as to feed a rod portion having a diameter greater than the inner diameter of the heating device into the melting zone, forming a diametral constriction in the melting zone, and after forming the rod portion to the specific cross section, displacing the end holders relative to the heating device in a vertical direction only.

2. Method according to claim 1, wherein the rod portion between the one end holder and the molten zone is formed to a diameter substantially the same as the diameter of the rod portion being fed to the molten zone.

3. Method according to claim 1, wherein the annular heating device is an induction coil having a diameter smaller than that of both rod portions and the molten zone is inductively heated by the coil.

4. Method according to claim 1, which includes afterheating the rod portion formed to the specific diameter.

5. Method according to claim 1, which includes preheating the rod portion being fed to the melting zone.

6. Method according to claim 1, which includes initially fusing to one of the ends of the rod a seed crystal having a smaller diameter than the inner diameter of the annular heating device. 

2. Method according to claim 1, wherein the rod portion between the one end holder and the molten zone is formed to a diameter substantially the same as the diameter of the rod portion being fed to the molten zone.
 3. Method according to claim 1, wherein the annular heating device is an induction coil having a diameter smaller than that of both rod portions and the molten zone is inductively heated by the coil.
 4. Method according to claim 1, which includes afterheating the rod portion formed to the specific diameter.
 5. Method according to claim 1, which includes preheating the rod portion being fed to the melting zone.
 6. Method according to claim 1, which includes initially fusing to one of the ends of the rod a seed crystal having a smaller diameter than the inner diameter of the annular heating device. 